Typically the general purpose microprocessor may be constituted by an Intel 8088 processor and the dedicated graphics microprocessor by an Intel 8051 processor. Both processors share a common random access memory or buffer in such a manner that graphic orders received at the display apparatus by the general purpose processor are passed to the dedicated processor via the shared memory to be converted, in conjunction as necessary with the special purpose hardware into the bit pattern to be stored in the refresh buffer. The general purpose processor may either receive high-level graphic orders which it converts into low-level graphic orders for the graphic processor or it can also receive low-level graphic orders which it passes unchanged to the graphics processor.
The two processors write asynchronously in a producer/consumer relationship, communication being achieved via a queue or "pipeline" between the two processors. The first process, that is that performed by the general purpose processor generally runs much slower than the second so that the queue is usually empty. Chapter 10 (see in particular FIG. 10.17) of the book "Fundamentals of Interactive Computer Graphics" edited by Foley and Van Dam, published by Addison-Wesley Pubishing Company, 1982, describes a two-processor pipelined architecture for a graphical display.
Where the two processors are linked by a pipeline which is generally empty, flicker can occur when part of the graphics image or picture is moved across the display screen. Examples of such image movement include the use of a moving cursor or changing the magnitude or orientation or position of a displayed object. The flicker occurs because it takes some time to compute how the old picture is to be processed to remove it from the display, to change the picture description and to process the new description into the display. If the old image is removed before the new one is processed, the screen will contain no "echo" for one picture process time period and the time required to change the description. This can be perceptible to the human eye resulting in flicker.
One solution is to use two refresh buffers, processing new images into them alternately and switching between the refresh buffers when the new image is complete. Clearly this adds to the cost of the display apparatus since the whole refresh buffer (possibly 3 or 4 Megabits in size) has to be duplicated together with some complication in buffer accessing. Alternatively, small images may be merged by the video refresh logic of an arbitrary point on the display. This requires extra video logic and is constrained in the shapes that can be displayed: aforementioned application Ser. No. 639,760 generates cross-hair cursor elements in this manner.